1. Field of the Invention
Embodiments of the present invention relate to a memory device, and preferred embodiments relate more particularly to a memory device having nonvolatility, low price, high integration density and low power consumption characteristics, which comprises a dendrimer as a material having memory characteristics between an upper electrode and a lower electrode, in which the dendrimer contains at least one electron-donating group and at least one electron-accepting group.
2. Description of the Prior Art
Recently, with the remarkable development of the information communication industry, the demand for various memory devices is rapidly increasing. Particularly, memory devices required in handy terminals, various smart cards, electronic cash, digital cameras, MP3 players, etc., require nonvolatility, meaning that even when the power is turned off, the written information is typically not erased.
With the development of large-scale integration (LSI) technology, the number of memory bits in an IC chip reaches the megabit level so that lines and spaces on the submicron order are required. Most existing nonvolatile memories are based on the standard silicon processing, but these silicon-based devices have a problem in the realization of high capacity because their structure is complicated, making the size of one memory cell large. Also, for these silicon-based memories, high memory capacity can be obtained only by a microfabrication process of reducing line and space per unit area, in which case it is expected that it will be faced with the problem of non-profitability because an increase in process cost will lead to an increase in the production cost of memory chips, and chips can no longer be miniaturized due to technical limitations.
Accordingly, the development of next-generation memory having ultrahigh speed, large capacity and low powder consumption characteristics, which is capable of substituting for the existing memory and is suitable for the development of portable information communication systems and devices for wirelessly processing large-amount of information, is being actively conducted.
The next-generation memory is divided, according to the material of a cell, a fundamental unit within a semiconductor, into ferroelectric RAM, magnetic RAM, phase change RAM, nanotube RAM, holographic memory, organic memory, etc.
Among them, organic memory exhibits memory characteristics using bistability shown when applying voltage to an organic material sandwiched between upper and lower electrodes. Such organic memory can realize a nonvolatile property, the advantage of existing flash memory, while overcoming the problems of processibility of existing flash memory, production cost and integration density noted as shortcomings, and thus, they are receiving great attention as the next generation memory.
In 1979, Potember et al. first reported electrical switching and memory phenomena at the nanosecond speed using CuTCNQ (7,7,8,8-tetracyano-p-quinodimethane), an organometallic charge transfer complex [Appl. Phys. Lett., 34 (1979) 405]. Japanese patent laid-open No. Sho 62-956882 discloses an electrical memory device including CuTCNQ, etc. This memory device has no advantage in terms of a process since it cannot be fabricated by means of a simple method, such as spin coating, instead it can be fabricated only by thermal evaporation with the use of an expensive vaporizer, due to the use of single molecules.
Organic materials known to show electrical bistability upon the application of an electric field include, in addition to the charge transfer materials, conductive polymers [Thin Solid Film 446 (2004) 296-300]. Also, memory characteristics using a phthalocyanine-based compound, an organic dye, were reported [Organic Electronics 10 4 (2003) 3944]. Also, switching/memory characteristics using a conformational change in an oxidation-reduction reaction and an electric field are known [Applied Physics Letter 82 (2003) 1215].
U.S. patent publication No. 2002-163057 discloses a semiconductor device comprising an interlayer between upper and lower electrodes, in which the interlayer is formed of a conductive polymer containing an ionic salt, such as NaCl or CsCl. This device shows switching/memory characteristics using a charge separation phenomenon caused by an electric field. However, while the conductive polymer can be spin-coated, but it is difficult to realize precise molecular weight and distribution thereof, leading to a problem in the reproducibility of material characteristics. Thus, uniform device performance cannot be realized.
U.S. Pat. No. 6,055,180 discloses a memory device using ferroelectricity caused by the crystalline state of a fluorine-based polymer, such as poly(vinyldifluoroethylene). However, the fluorine-based polymer has a problem in that it is difficult to coat due to the hydrophobic property of fluorine, leading to a reduction in processibility. Another problem is that recording of information is possible only one time and the reading of stored information is optically conducted, leading to an increase in the size and complexity of devices.